In recent years, for example, magnetic disk drives have become widely used as large-capacity disk drives used in electronic apparatuses, such as personal computers. In general, a magnetic disk drive comprises a magnetic disk in a case, spindle motor configured to support and rotate the disk, pivotable head stack assembly (HSA) that supports magnetic heads, voice coil motor (VCM) that drives the HSA, circuit board unit, etc. The HSA comprises a plurality of head gimbal assemblies (HGAs) supported on arms, individually.
Each HGA comprises a suspension, mounted on the distal end portion of an arm, and a magnetic head supported on the suspension. The suspension comprises a load beam and a baseplate secured to the proximal end side of the load beam and the distal end portion of the arm.
The baseplate as a component of the HGA is generally fastened to the arm by ball swaging for the sake of repairability. Fastening by ball swaging is a method in which an annular protrusion formed on the baseplate is crushed against the arm and plastically deformed so that these two portions are joined together. In doing this, a ball larger in diameter than a bore of the protrusion is passed through the protrusion bore with the protrusion fitted into a circular opening in the arm.
The flying height of the head in modern magnetic disk drives is adjusted to more than 10 nm for high-density recording. Therefore, the displacement of the baseplate that occurs when the ball is swaged adversely affects the flying height and loading-unloading characteristics of each magnetic head. In the case where the baseplate is located on a swaged seat surface formed on one surface side of the arm, for example, the protrusion of the baseplate is plastically deformed by the ball and fastened to a swaged hole of the arm. As the protrusion is thus plastically deformed, the baseplate is warped. In addition to this, the swaged seat surface of the baseplate is also displaced by pressure from the protrusion of the baseplate. As the arm is displaced in this manner, the distal end of the baseplate is also greatly displaced. In the case where the baseplate is located on only one side of the arm, the displacement of the arm mainly causes the displacement of the distal end of the baseplate. If the arm and baseplate are displaced in this manner, the variation of the flying height of the magnetic head supported on the suspension is affected. Thus, the displacement of the distal end of the baseplate is expected to be reduced to improve the reliability of an HDD.
In order to improve the displacement of the distal end of the baseplate, the fastening force between the baseplate and arm may be reduced or the rigidity of the arm may be improved to reduce the displacement of the arm. If the fastening force is reduced, however, the possibility of disengagement of the baseplate increases during operation or if the HDD is jolted. Thus, the fastening force can be reduced only limitedly. Although the arm can be made more rigid by thickening its swaged seat surface, moreover, it then inevitably becomes heavier.